1) Field of Invention
This invention relates generally to voltage regulation devices and, more particularly to pulse width modulation.
2) Description of Related Art
Pulse width modulation has been employed for a number of years in order to attain high efficiency voltage regulation. There are, however, a plurality of drawbacks when utilizing pulse width modulation, in lieu of linear regulation in order to stabilize output voltage.
Pulse width modulators generate both input and output noise not present in linear regulation. A pulse width modulator's efficiency is primarily due to the fact that the output voltage is either at substantially the input voltage or substantially at ground potential. The fast switching between the two states, necessary in order to achieve high efficiency regulation, also generates input noise as input current is switched to and from the load. Low switching times also contribute to emitted noise at the system output.
Pulse width modulators also tend to be more complex than linear regulators, particularly when an inductive load is to be supplied. Whenever an input current to a load which has substantial inductance is eliminated, an opposing voltage is generated by the load. Two means have been employed in order to limit the magnitude of the opposing voltage. A snubbing diode, though simple to implement, limits the opposing voltage to approximately one volt below ground potential during the off state of current supplying switch. During this time a current, substantially equal to the current supplied while the current supplying switch conducts, passes through the diode. This current dissipates heat and limits the efficiency of the modulator. Synchronous modulation, whereby a second switch is accurately timed to shunt the opposing voltage to ground, is more efficient, but complex and accurate timing circuits are needed for implementation.
An input source with substantial inductance will also produce an opposing voltage of the same polarity as the supply source when the current to the load is eliminated. Input filtering is generally utilized in order to limit the opposing source voltage. The input filter adds substantial cost to the system and limits spatial efficiency. Non-adaptive open loop frequency limiting components have been utilized in order to increase the system switching times in order to reduce output noise emissions and to alleviate the requirements of the input filtering system. Because the filtering does not adapt to source and load elements, modulator efficiency is reduced over varying conditions.
In order to efficiently switch current to the load, an on board voltage multiplier is typically employed in order to energize the current passing switch. A typical multiplier consists of an oscillator, a rectifier circuit, and a filtering circuit, adding complexity to the modulator and limiting spatial efficiency.